In a later tweet, they clarified that the 18+ rating was "in accordance with the law number 436-FZ." You're probably not a Russian Federation law aficionado so I'll explain.

The law in question is called "On Protecting Children from Information Harmful to Their Health and Development." It was originally intended to prevent minors from seeing explicit material such as violence, self-harm or drug abuse. However, last year, it was amended to restrict information "[aimed] at causing minors to form non-traditional sexual predispositions, notions of attractiveness of non-traditional sexual relationships, distorted ideas about the equal social value of traditional and non-traditional sexual relationships, or imposing information about non-traditional sexual relationships which raises interest in such relationships insofar as these acts do not amount to a criminal offence."

Characters in The Sims 4 can form same-sex relationships so, according to this law, it must be kept out of the hands of minors. After all, who knows what will happen if they see two virtual characters of the same virtual gender paired off. It might make them think one of these dangerous thoughts:

"These Sims are gay. Because I have no control over my own actions and mindlessly imitate anything I see in video games, I will become gay as well."

"Huh. These Sims are gay and it didn't destroy the entire Sims universe. I guess gay people aren't so bad after all."

"Hey, these Sims are gay and they don't seem weirder or less moral than straight Sims. Maybe the government is just using gays as a scapegoat to score political points and distract us from more important issues."

Ars Technica points out that the rating might not actually affect sales at all. Piracy is very prevalent there so the game might spread far and wide throughout the country whether or not the government likes it. The lack of always-online DRM should help these pirates.

EA hasn't commented on the rating yet. I imagine they're not too pleased with the situation, though. EA was named one of the best places to work in America by the Human Rights Campaign because of their support of lesbian, gay, bisexual and transgender employees. It seems unlikely that they'd remove same-sex relationships from the game in the Russian version of the game so the rating will probably stand.